If you board a train in Baltimore, headed to Washington, D.C., you'll pass over the Thomas Viaduct, a stone bridge built 180 years ago. If you pour a glass of water in Chicago or Philadelphia, it might reach you through century-old pipes. Fly into Minneapolis or Albany, New York, and you will land at an airport built in the early days of aviation.

The infrastructure decisions Americans made in the past echo today. Infrastructure investment has connected the people and industries of our vast continent with rails and roads, and it has saved lives in our cities with modern water and sewer systems.

The stakes in the current infrastructure debate are high. But what matters most is not the size of any federal infrastructure package, nor how it is financed, nor even how many jobs it creates in the coming years. What matters most is building the infrastructure that will enable America to respond to the challenges and opportunities of the 21st century.

Any infrastructure package that incentivizes the wrong investments or unwisely rolls back crucial environmental, public health and safety regulations in the name of short-term profits could waste taxpayer money or, even worse, hurt the nation's future health and well-being.

The New Deal infrastructure programs of the 1930s are famous for putting unemployed Americans back to work, but that isn't all they achieved. Back then, economic opportunity was constrained by lack of access to good roads and electricity across much of the country. The iconic infrastructure of the New Deal -- from the Merritt Parkway in Connecticut to the power systems of the Tennessee Valley Authority -- addressed those needs, helping to position the nation for 20th century success.

What is needed to advance America now?

Answering that question begins with knowing what we don't need to build. America already has too much infrastructure that we don't maintain adequately, or which we don't use to its fullest potential. Research indicates that fixing existing roads, for example, delivers greater return on investment than expanding them. We don't need new pipelines to distribute even more dangerous fossil fuels nearly as much as we need to fix existing pipeline leaks.

And we should prioritize stewardship of our existing "green infrastructure" of wetlands and forests -- the natural lands that absorb floodwaters and filter out pollutants before they reach our rivers, streams and lakes. Doing so will minimize the cost of the water and sewer system upgrades we need to deliver safe drinking water to our families and keep our rivers and lakes clean.

When we do build, we need to keep 21st century priorities front and center, and incorporate the revolutionary knowledge and technologies spawned over the decades since America's last infrastructure overhaul.

As more households transition from just consuming energy to also producing energy with solar panels and other renewable technologies, we will need to build a modern, decentralized electric grid to accommodate them. Rural Americans today need access to broadband Internet the way they once needed electricity. As Americans, especially millennials, look for new transportation options without the stress and cost of driving, we need to provide them with new choices, such as convenient public transportation. And since whatever we construct today will be expected to last for a long time, our investments must make sense in an era of climate change.

Moreover, we need to make sure that the money we spend -- either in the form of direct investment or tax breaks -- results in infrastructure that is managed for the public interest. Conducting a fire sale of our current publicly owned assets to finance a new wave of infrastructure construction, or subsidizing the construction of infrastructure for private benefit, could leave America weaker, not stronger, as we face the challenges of the modern era.

With a smart approach to infrastructure that is grounded in 21st century needs, our children and grandchildren can look back with gratitude at the wisdom of our choices. Without one, they will look back in disappointment at the precious opportunities -- not to mention dollars -- we wasted.